No idea where the assumption that Shimon Ben Shotoch did not convene a Beis Din to determine if the women were guilty or not is based on. That it is not the impression one gets from learning the sugya where the story is discussed. It is true that the story of the witches did not go in accordance with the normal procedures of misas beis din (for reasons discussed in the Gemora and Rishonim) but
it seems pretty clear that he did determine their guilt.
Even those involved in the Salem with trials themselves untimely came to agree that it was aterrible travesty.